WASHINGTON — Republicans narrowly advanced their campaign to roll back the Affordable Care Act on Tuesday, as the Senate voted by a slim margin to begin debating legislation to repeal and potentially replace large sections of the 2010 law signed by President Barack Obama.

But the partisan 51-50 vote — with Vice President Mike Pence breaking a tie — does not assure the success of the GOP’s seven-year quest to dismantle the sweeping law, often called Obamacare.

With health coverage for tens of millions of Americans at stake, it remained unclear Tuesday what kind of health care bill — if any — might emerge by the time a final Senate vote is held, possibly as early as Thursday.

Senate Majority Leader Mitch McConnell, R-Kentucky, has set up a series of votes this week on competing proposals to repeal much of Obamacare, or repeal and replace pieces of the law.

Tuesday’s procedural vote to kick off that process marks a victory for McConnell, who has been laboring for months behind closed doors to rally his divided caucus, and for President Donald Trump, who has been publicly calling out GOP senators to stick with the repeal push.

But to get to the vote, McConnell abandoned customary legislative process, foregoing public hearings and committee debate in a way almost never seen for major legislative proposals.

And he left Republican lawmakers with a series of legislative options — which polls show are also deeply unpopular with Americans — that would leave as many as 32 million more people without health coverage and weaken health protections for tens of millions more.

The GOP plans have been widely panned by independent analysts and vigorously opposed by every major patient advocacy group and every leading organization representing physicians, nurses and hospitals.

Opponents include the American Diabetes Association, the March of Dimes, the AARP, the American Medical Association, the American Lung Association, the National Multiple Sclerosis Society, the American Heart Association and the advocacy arm of the American Cancer Society.

Republican Sens. Susan Collins of Maine and Lisa Murkowski of Alaska voted against the motion.

Collins posted on Twitter on Tuesday afternoon that she voted against the motion to proceed with debate on the bill because “when dealing w/ a complex issue that affects millions of Americans & ⅙th of our economy, we must proceed carefully.”

“Making sweeping changes to the 50-year-old Medicaid program without a single Senate hearing is a mistake,” she tweeted. “We must work in a bipartisan way to to fix the ACA’s serious flaws.”

In a statement Tuesday, Maine’s junior senator, independent Angus King, called the legislation “misguided,” adding that any version of the bill “will hurt millions of people across the country and tens of thousands of hardworking people across Maine.”

“I’ve never seen a process like this before with no hearings and it will almost certainly produce a terrible bill that will drastically raise costs for people in Maine, put coverage out of reach for many others, and reduce Medicaid funding, causing states to choose between helping seniors or children or people with disabilities,” King said. “As the Senate advances with this bill, I will continue to support and protect people in Maine and work to find solutions that keep people covered and make health care more affordable.”

GOP leaders were aided by the dramatic return to the Capitol by Sen. John McCain, R-Arizona, after surgery last week and a diagnosis of brain cancer. He arrived in the chamber to cheers and a standing ovation from his colleagues.

As Republicans next move to crafting a final bill, one leading GOP option, favored by conservatives, is to simply repeal most major parts of Obamacare, and then try to pass a new health care law in the future.

Another idea, supported at one point by GOP leaders, would replace most of the law while at the same time dramatically scaling back federal funding of the Medicaid insurance program that covers some 74 million low-income, elderly and disabled Americans.

GOP centrists have complained both of those versions would leave too many Americans without insurance, and have urged McConnell to start the process over, working on a bipartisan approach that will protect those covered under Obamacare.

So far, none of these approaches has garnered the necessary 50 votes from the Republicans’ 52-seat Senate majority.

The lack of GOP consensus prompted Senate Republican leaders to float a new idea Tuesday — a far more limited repeal bill that would target the most unpopular parts of Obamacare, including its insurance mandates.

That plan — dubbed “skinny repeal” — would eliminate only a medical device tax established by Obamacare and two mandates requiring Americans to have coverage and requiring large employers to offer health benefits.

The approach would fall far short of what Republicans have been promising for years.

If it passes, Senate Republicans could at least send something to a conference committee where lawmakers could work out a compromise with a sweeping repeal bill passed by House Republicans in May, giving them yet another shot at unwinding Obamacare.

That would all have to be done without Democrats, who remain fiercely opposed to the GOP repeal push and voted together Tuesday to try to block the beginning of Senate debate.

The skinny repeal plan risks major disruptions to the nation’s insurance markets, as the lack of any insurance mandate could free Americans to buy health insurance only when they get sick, driving up costs.

But it would leave in place other critical parts of the 2010 law, including hundreds of billions of dollars of federal aid to state Medicaid programs and insurance subsidies that help low- and moderate-income Americans

That assistance is credited with helping extend coverage to more than 20 million previously uninsured Americans.

Also untouched would be landmark rules established by Obamacare that require insurers to cover a basic set of benefits and prohibits health plans from discriminating against Americans with preexisting medical conditions.

It remains unclear whether House Republicans would agree to this more limited repeal plan.

But with centrist and conservative GOP senators in deep disagreement, the skinny repeal may be all McConnell can pass.

The repeal-only plan — which is favored by the most conservative GOP senators — would leave an estimated 32 million more people without health coverage over the next decade, according to the nonpartisan Congressional Budget Office, or CBO.

It is opposed by multiple centrist GOP senators.

The alternative proposed by McConnell that repeals and replaces major parts of the law while also dramatically scaling back federal Medicaid assistance is only slightly less disruptive, CBO analysts have concluded.

The McConnell proposal, which has drawn fire from both centrist and conservative Republicans, would leave 22 million more Americans without insurance by 2026.

That plan would be most devastating to the Medicaid safety net, which currently covers some 70 million poor Americans, including children. Federal Medicaid funding would be reduced by more than a third over the next two decades.

And while CBO concluded the “repeal-and-replace” plan could lower premiums for some consumers, it would likely put health insurance out of reach for older and sicker Americans who could be forced to pay larger deductibles or higher premiums than under the current law.

Still unclear is the effect of yet other GOP proposals that could come up for votes in coming days, including one by Sen. Ted Cruz, R-Texas, to allow health insurers to sell skimpier plans that do not include basic benefits such as prescription drugs, maternity care and mental health and substance abuse services.

The Cruz proposal is widely expected to make health coverage more expensive for sick Americans who need to buy more extensive health coverage and has been panned by health insurers as “unworkable.”

But McConnell elected to proceed with voting Tuesday before CBO had an opportunity to assess the potential impact of Cruz’s proposal.